Illumination systems of said type are sufficiently well known in the prior art, the simplest method of operating LEDs consisting in connecting the LED to a DC source together with a series resistor. The series resistor is used to limit the light-emitting diode current IF (conducting-state current). The conducting-state current IF can be varied by varying the series resistor, as the result of which the brightness of the light emitted by the light-emitting diodes is likewise varied. It is possible in this simple way to achieve the dimmability of a system that can comprise light-emitting diodes and other luminous means as light source. However, one problem consists in that with some LEDs, in particular with InGaN LEDs, there is a more or less strong relationship between the current intensity of the conducting-state current IF and the associated wavelength of the emitted light. Depending on the conducting-state current IF, a shift occurs here in wavelength or color locus, that is to say when dimming there is a simultaneous change in the wavelength, and thus in the color, of the emitted light.
Re color locus: In the case of the standard valence system, a color is described as the sum of three mixing values, the so-called standard color values X, Y, Z (DIN 5033). The standard color value components Cx and Cy are frequently also specified for the purpose of two-dimensional representation, in which case Cx=X/(X+Y+Z) and Cy=Y/(X+Y+Z). In the case of the graphic representation of chromaticity in a two-dimensional chromaticity diagram, Cx and Cy serve as rectangular coordinates of the so-called color loci.
The phenomenon of color locus displacement can be observed most impressively with green LEDs. With type LT E673 (power TOPLED), the dominant wavelength is above 540 nm given a conducting-state current of 3 mA, and drops below 512 nm when the conducting-state current rises to 90 mA. With an LED of type LW W5SG (dragon), the color locus changes in a fashion defined by Cx and Cy from Cx=0.322 and Cy=0.316 for a conducting-state current of 100 mA to Cx=0.316 and Cy=0.301 for a conducting-state current IF=1000 mA.
The principle of pulse width modulation is applied in the prior art in order to solve this problem, that is to say to keep constant the color locus of the light emitted by the LED. In this case, an LED is switched on and switched off again over a period T always with the same conducting-state current IFmax. The pulse duty factor, which is calculated from the quotient of the switch-on time tp divided by the period T, determines the brightness of the light emitted by the LED. A large pulse duty factor leads to a brightly shining LED, or conversely a small pulse duty factor leads to a more weakly shining LED. In this case, the integration of the light emitted by the LED is undertaken by the human eye. For periods T shorter than 10 ms, the light is registered as continuous by the eye. Flickering of the light can be perceived by the eye in the case of longer periods.
This driving known from the prior art poses no problem in the case of low powers. Upon transition to higher powers, however, as is required in the case of lamps for general lighting or use in a motor vehicle, undesired interference can arise. Two types of interference essentially come into consideration in this case, specifically radio interference (EMC) and conducted interference.
In order to prevent such electromagnetic interference, the appropriate standard from BMW provides that the rising edge is at most 20 mA/μs in clocked operation. If this limit is adhered to, this results in very long switch-on and switch-off times and, in association therewith, very high switching losses. Thus, with the measures known from the prior art, a decision has to be made for one of the evils, either interference or color locus displacements or high switching losses.